1. Field of the Invention
The present invention relates to an optical fiber cable laid underground, on the ground, overhead, or undersea and a method of making the same; and, in particular, to an optical fiber cable with jacket in which a plurality of optical fibers are assembled as being S-Z stranded, and a method of making the same.
2, Related Background Art
There has conventionally been known an optical fiber cable with a jacket in which a plurality of optical fibers are assembled as being S-Z stranded about a central member embedding therein a tension member. Such an optical fiber cable is laid underground, on the ground, overhead, or undersea. There are cases where, even after the optical fiber cable is once laid, it is necessary to take out and branch a part of the optical fibers included in the optical fiber cable by removing or ripping a part of the jacket in an intermediate portion of the optical fiber cable (so-called late intermediate branching).
Here, in an optical fiber cable containing therein a plurality of optical fibers extending in their S-Z state, the length of each contained optical fiber would be longer than the length of the optical fiber cable itself (which is identical to the length of-the central member). Therefore, if the jacket is removed from near a reverse portion of the S-Z stranded line formed by an optical fiber (referring to the portion in which the optical fiber reverses from S-strand to Z-strand or vice versa), the optical fiber can smoothly be taken out from the optical fiber cable without being pulled in excess. In view of this point, the workability at the time of branching after laying the cable will improve if the reverse portion of the S-Z stranded line formed by the optical fiber is discernible from the outside of the jacket of the optical fiber cable.
As techniques relating to the above-mentioned matter, those disclosed in U.S. Pat. Nos. 4,828,352 and 5,729,966 have been known. In the conventional optical fiber cables described in these publications, a plurality of optical fibers (optical fibers or optical fiber ribbons) are assembled as being S-Z stranded about a central member. Thus assembled optical fibers are surrounded with a jacket made of a synthetic resin or the like. The jacket is provided with a reverse-portion-indicating mark, such as a symbol, letter, or the like, at the position corresponding to a reverse portion of the S-Z stranded line formed by each optical fiber.
Such an optical fiber cable is made according to the following procedure. Namely, in the case where the steps of stranding optical fibers to forming the jacket are carried out by a single manufacture line, while a core marker apparatus is disposed downstream of a jacket-cooling water bath. Also, a signal indicative of the reversing direction of the lay plate located on the most downstream side among a plurality of lay plates used for stranding the optical fibers about the central member is taken out. When it is determined according to this signal, the feeding length of the central member, and a predetermined offset length that a reverse portion has reached the core marker apparatus, then a reverse-portion-indicating mark is formed on the jacket by the core marker apparatus.
In the case where the step of stranding the optical fibers and the step of forming the jacket are separated from each other, on the other hand, the position of the reverse portion is determined in the step of stranding the optical fibers according to a process similar to that in the above-mentioned case where the optical fiber cable is made by a single manufacture line, and then a color tape, metal tape, or the like indicative of the position of the reverse portion is attached onto a wrapping binder tape wound about the outer periphery of each optical fiber. In the step of forming the jacket, the color tape or the like attached on the wrapping binder tape is detected by a color sensor, metal sensor, or the like before extrusion-molding the jacket, and a reverse-portion-indicating mark is formed on the extrusion-molded jacket according to thus detected signal.